Electrochemistry (12)

MCAT General Chemistry Review 2025-2026

Electrochemical Cell

describes any cell in which oxidation-reduction reactions take place. Certain characteristics are shared between all types of these

Electrodes

are strips of metal or other conductive materials placed in an electrolyte solution

Anode

is always the site of oxidation. It attracts anions

Cathode

is always the site of reduction. It attracts cations

Electrons

flow from the anode to the cathode

Current

flows from the cathode to the anode

Cell Diagrams

are shorthand notation that represent the reactions taking place in an electrochemical cell

Anode to Cathode

cell diagrams are written from __________ with electrolytes (the solution) in between

Vertical Line

a _______ in a cell diagram represents a phase boundary

Double Vertical Line

a _______ in a cell diagram represents a salt bridge or other physical boundary

Galvanic (Voltaic) Cells

house spontaneous reactions (delta G < 0) with a positive electromotive force

Electrolytic Cells

house nonspontaneous reactions (delta G > 0) with a negative electromotive force. These nonspontaneous cells can be used to create useful products through electrolysis

Concentration Cells

are a specialized form of a galvanic cell in which both electrodes are made of the same material. Rather than a potential difference causing the movement of charge, it is the concentration gradient between the two solutions

Negatively; Positively

for galvanic cells, the anode is _________ charged and the cathode is _________ charged

Positively; Negatively

for electrolytic cells, the anode is _________ charged and the cathode is _________ charged

Rechargeable Batteries

are electrochemical cells that can experience charging and discharging states. These are often ranked by energy density

Charging

electrolytic state of rechargeable batteries

Discharging

galvanic state of rechargeable batteries

Energy Density

the amount of energy a cell can produce relative to the mass of battery material

Lead-acid Batteries

when discharging, consist of a Pb anode and a PbO2 cathode in a concentrated sulfuric acid solution. When charging, the PbSO4- played electrons are dissociated to restore the original Pb and PbO2 electrodes and concentrate the electrolyte. These cells have a low energy density

Nickel-cadmium Batteries (Ni-Cd)

when discharging, consist of a Cd anode and a NiO(OH) cathode in a concentrated KOH solution. When charging, the Ni(OH)2 and Cd(OH)2 plated electrodes are dissociated to restore the original Cd and NiO(OH) electrodes and concentrate the electrolyte. These cells have a higher energy density than lead-acid batteries

Nickel-metal Hydride Batteries (NiMH)

have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective, and are significantly less toxic

Surge Current

is an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved

Reduction Potential

quantifies the tendency for a species to gain electrons and be reduced. The higher this is, the more a given species wants to be reduced

Standard Reduction Potential (Eo red)

are calculated by comparison to the standard hydrogen electrode (SHE) under the standard conditions of 298 K, 1 atm pressure, and 1 M concentrations

0 V

the standard hydrogen electrode has a standard reduction potential of ___

Standard Electromotive Force (Eo cell)

is the difference in standard reduction potential between the two half-cells

Positive

for galvanic cells, the difference of the reduction potentials of the two half-reactions is __________

Negative

for electrolytic cells, the difference of the reduction potentials of the two half-reactions is __________

Positive; Negative

when Eo cell is _________, delta Go is _________. This is the case in galvanic cells

Negative; Positive

when Eo cell is _________, delta Go is _________. This is the case in electrolytic cells

0; 0

when Eo cell is _________, delta Go is _________. This is the case in concentration cells

Nernst Equation

describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force

Keq

the ratio of products’ concentrations at equilibrium over reactants’, raised to their stoichiometric coefficients

Positive

when Keq is greater than 1, Eo cell is _________

Negative

when Keq is less than 1, Eo cell is _________

0

when Keq is equal to 1, Eo cell is _________